1. Field of the Invention
An aspect of the present invention relates to an optical scanning apparatus in an electro-photographic type image forming device, and more particularly to an optical scanning apparatus that drives a plurality of laser beams at the same time.
2. Description of the Related Art
In the electro-photographic type image forming device such as a laser printer, it has been desired that two or more kinds of a print line width or a print dot density of an image can be selected in one image forming device.
In the image forming device of one beam scanning type, the rotating speed of a rotating polygon mirror is changed to change a dot space in a sub-scanning direction and a light intensity modulating clock frequency of a light source is changed to change a dot space in a scanning direction so that the print dot density can be changed.
However, in recent years, a high speed and high density rapidly progress and a multi-beam scanning system is used that a plurality of optical beams are driven at the same time during one scanning operation. As the light source, since a semiconductor laser array or a two-dimensional face light emitting laser is used, the print dot density can not be changed in the usual system. In the multi-beam scanning system, since a plurality of semiconductor laser elements are obliquely arranged or arranged lengthwise to scan a photo-sensitive member, the space of the optical beams in the sub-scanning direction is set to a fixed value in accordance with the arrangement of the semiconductor laser elements. Accordingly, even when the rotating speed of the rotating polygon mirror is changed as described above, since the space between the beams outputted from the light source in the sub-scanning direction is fixed during the one scanning operation, a print density can not be changed.
JP-A-9-159957 discloses an optical scanning apparatus having a light source composed of a semiconductor laser, a collimator lens that collimates light from the light source and a cylinder lens that guides the light from the collimator lens to a rotating polygon mirror, wherein two lenses that can be freely taken in and out from an optical path are provided between the collimator lens and the cylinder lens.
JP-A-10-217544 and JP-A-10-161047 disclose that a light source is mechanically rotated to change an inclination so that a space between a plurality of beams in a sub-scanning direction is changed on a photo-sensitive member. In these systems, when a dot density is changed, the rotating speed of a rotating polygon mirror is also changed.
JP-A-2-206566 discloses a device in which the rotating speed of a spindle motor 12 (for a rotating polygon mirror) is varied to change a resolution, however, since a switching time is necessary due to an inertia, when the least common multiple of N kinds of resolutions, for instance, the resolutions of 240 dpi, 360 dpi, 480 dpi and 960 dpi is required, the spindle motor is rotated at such a rotating speed as to obtain the resolution of 2880 dpi as the least common multiple thereof and the resolution is switched by a resolution switching signal determined by a print magnification supplied from a host device, and a print start signal (BD) is switched to a signal obtained by dividing a frequency at an integer ratio in accordance with the resolution to repeat an exposure in that cycle.
In JP-A-9-159957, JP-A-10-217544 and JP-A-10-161047, since a mechanism for taking in and out the lens to the optical path or a mechanism for rotating the light source are necessary, a problem arises that the structure of the optical scanning apparatus is complicated. Further, a mechanical accuracy is required for adjusting a beam pitch at intervals of several μm, so that a problem also arises that the optical scanning apparatus is expensive.
Further, when the dot density is changed, the rotating speed of the rotating polygon mirror is also changed. For instance, when the dot density is decreased to half of the dot density, the rotating speed of the rotating polygon mirror also needs to be half of the rotating speed. It takes several ten seconds to several minutes to change the dot density. Therefore, for instance, change of the dot density can not be finished between pages (in several ten ms to several hundred ms) In JP-A-2-206566, a necessary dot density can be easily realized without changing the rotating speed of the polygon mirror. However, in this system, since a beam is detected and the frequency is divided to switch the resolution, a beam space for scanning the BD is extremely narrow to make it difficult to separate adjacent beams and generate the BD signal corresponding to the dot density in a plurality of beam scanning operation using a laser array light source with a plurality of laser elements arranged at intervals of narrow pitches.